The "Na ion gradient hypothesis" proposes that in animal cells the electrochemical potential gradient for Na ion, which is established by active Na/K transport, serves as the sole energy source for driving accumulation of certain organic solutes by the cell. The major controversy about this postulate is the adequacy of the energy available from the cation gradients to totally energize amino acid uptake. This proposal is based on the hypothesis that the cell membrane potential plays an essential role in both the regulation of cellular electrolyte activity and the provision of nutrients necessary for cellular metabolism. The long-range goal of the project is to characterize the role of the membrane potential as an important component of nutrient uptake, and in particular amino acid transport, essentially for maintenance of cell integrity and growth. To accomplish this objective we will investigate in Ehrlich ascites tumor cells: (1) the effect of amino acid transport on the cell membrane potential; (2) the effects of amino acids on the intracellular Na ion and K ion electrochemical activity; (3) the coupling stoichiometry of cation-amino acid co-transport; and (4) how experimental alterations of the membrane potential affect amino acid transport. Membrane potential and cytoplasmic cation activities will be measured with intracellular, glass microelectrodes. We anticippte that this investigation will define the role of the cell membrane potential in the regulation of amino acid movements and lead to a more complete understanding of the energetics involved in meeting the cell's nutritional requirements.